The present invention relates to a line circuit for network line junctor adapted to be inserted in private switching equipment or in subscribers' terminals.
A network line junctor is a device intended to link all the subscribers' lines of the same private autoswitch to a public telephone exchange by means of a two-wire network line. A network line junctor performs numerous functions, for instance transmitting the low frequency of the PTT line towards the link network of the private exchange or towards any other similar device (intercommunication set for example) and, vice versa, ensuring rejection of the common mode, ensuring good galvanic insulation of the PTT line with respect to earth, in particular.
These network line junctors comprise in particular a ringing detection device, a loop state control device, a dialling device.
The known dialling devices for network line junctor conventionally comprise an electromechanical relay introducing D.C. breaks in the loop in an appropriate number and at an appropriate frequency, said breaks then being detected by means of an electric circuit inserted on the public exchange side. Referring to FIG. 1, a conventional network line junctor or the prior art comprises a transformer 10 receiving the supply current from the public exchange and transmitting the low frequency to the private exchange 4.
Between the wires R.sub.A and T.sub.B of the network line, a capacitor 11 is generally disposed in series with a ringing detection circuit 13, connected by the normally closed contact 121 of the relay 12. The capacitor 11 makes it possible to couple the calling current device at rest and to protect the dialling relay 15. The work position of the contact 121 short-circuits the choke of the transformer 10 during dialling.
It was therefore usual, in order to limit the network line current, to insert a circuit 14 with three components placed in parallel: a capacitor 140, a resistor 142 and a thermistor 141. The latter thermistor 141 might be replaced by an assembly of contact-switchable resistors.
However, such dialling devices are not satisfactory due to a poor regulation of the D.C. supply. Relays are well known for their faults: bulky, noisy, expensive, and for their lack of reliability. The transformers is also very cumbersome as it must support the direct current.
French Pat. No. 2 251 975 in particular has already proposed a line circuit for line junctor comprising loop interrupting means particularly for dialling and comprising regulating means maintaining said loop current about a predetermined threshold, presenting a high impedance for the telephone frequencies and a low impedance for the ringing currents.
However, such a circuit limits the low frequency direct and alternating current, which is unfavorable. It does not operate in a sufficiently wide current-voltage range, and in particular at zero current. In addition, its functioning depends on a photothyristor which is a delicate component.
It is therefore an object of the present invention to improve such a circuit.